This invention relates to improvements in signal tube operated switches and particularly those switches for operation of an external circuit. One particular application is in the operation of delay detonators.
A relatively recent development in the explosives industry has been the use of signal tubes or shock tubes which are sold, for example, under the trade names Primadet, Nonel, hereinafter called signal tubes, for the initiation of detonators. Until the present time signal tubes have been used solely to provide a means of transmitting an initiating pulse from one detonator to another. Signal tube is ideally suited to this application because it is very resistant to accidental initiation, can propagate over an indefinite length and the detonation wave is fully contained within the tube.
Delay detonators are used extensively in the mining industry to initiate explosive charges in a predetermined sequence. There are two conventional types, electric and non-electric both of which consist of a metal (usually aluminium) tube (detonator tube) into which charges of a primary explosive (usually Lead Azide) and a secondary explosive (usually PETN) have been pressed. A length of pyrotechnic substance is then forced into the detonator tube on top of the explosive charge.
In an electric delay detonator the pyrotechnic substance is ignited by a small charge of pyrotechnic material located around some form of resistive wire or junction (usually referred to as a fusehead or ignition charge). Wires are attached to the terminals of this resistive wire or junction and are then connected to an electrical power supply usually referred to as a shot exploder.
The other is a signal tube detonator. In a non-electronic detonator the pyrotechnic substance is ignited by a signal tube which is typically of 3 mm outside diameter and 1 mm inside diameter; the inside of the tube is covered with a thin layer of explosive or reactive material and is attached to the detonator tube directed towards the pyrotechnic substance. Once initiated a shock wave will propagate through the tube and has sufficient energy to ignite the pyrotechnic substance.
There are a number of applications where one or other of the electric or non-electric delay detonator can only be used, for example, the signal tube type non-electric detonator cannot be used in gassy coal mines due to the danger of igniting the atmosphere. However, non-electric systems possess certain advantages including their ease of connecting up, very good tolerance to static electricity discharges and the ability to initiate a substantial number of subsequent charges in the blast once initiated by a single detonator or suitable initiator.
By comparison, the electric system is limited by two further factors; first, the number of initiating sequences is limited to the number of specific delay times that the manufacturer produces and, second, the shot exploder is only able to fire a given limited number of detonators. Both conventional types of detonator however, poses the disadvantage in that they have a limit of accuracy and precision of 1-2% which is viewed as less than desirable in most industries particularly the mining industry where greater control of rock fragmentation, ground vibration and improvement of overall efficiency is required. This is due to the inaccuracies inherent in the use of pyrotechnics as delay devices.
While there has been some development in the use of electronic delay devices and systems which would provide a limit of accuracy and precision of the order of 0.01-0.1%, there has been no device produced which combines or embodies the electronic delay timing system with the non-electric delay detonator system to maximise the overall efficiency of delay detonators.